Field of the Invention
The invention relates to a method for manufacturing composite particles, particularly composite particles based on titanium dioxide and carbonate, with the help of combined mixing, homogenization and precipitation in a pipeline mixer based on the rotor-stator principle.
Technological Background of the Invention
In paper goods, plastics, paints, coatings, adhesives, rubber, etc., it is customary to use fine, inorganic or organic fillers that serve as extenders and/or improve the technical properties of the matrix material. Different fillers are often used simultaneously in this context. The fillers are then present in the matrix either in a simple blend, or in the form of composite particles that are each composed of at least two different filler particles. Composite particles permit improved dispersion of the filler components in the matrix, and possibly further advantages.
For example, inorganic pigments, and particularly titanium dioxide pigments, are often incorporated into various matrices as whiteners, tinting agents or opacifiers. Owing to its high refractive index, titanium dioxide scatters light particularly efficiently and is therefore the most important white pigment for applications in paints and coatings, plastics, paper and fibers. The light-scattering efficiency declines if the titanium dioxide particles are distributed in the matrix at a distance of less than roughly half the wavelength of the light from each other, i.e. roughly 0.20 to 0.25 μm. The light-scattering efficiency is typically measured with the help of the hiding power or the tinting strength of the titanium dioxide pigment in the matrix.
On the other hand, titanium dioxide is a significant cost factor, and a search has been in progress for some time to find possibilities for reducing the quantity of titanium dioxide used, without having to accept significant losses of hiding power.
Savings are possible by combining titanium dioxide particles with suitable fillers that, as so-called “extender particles”, are intended to act as spacers for the TiO2 particles. The known manufacturing methods include both simple blending of the components and also the production of composite particles by bonding the TiO2 particles to the extender particles by means of a precipitated binder, or in-situ precipitation of the extender on the surface of the titanium dioxide particles.
Calcium carbonate is often used as an in-situ precipitated extender. In-situ precipitation is customarily performed by reacting calcium oxide or calcium hydroxide with carbon dioxide. To this end, an aqueous suspension of TiO2 particles or other mineral particles is mixed with CaO or Ca(OH)2, and CO2 gas is subsequently introduced, such that particulate calcium carbonate is precipitated (U.S. Pat. No. 5,082,887 A, WO 2000/078874 A1, EP 1 255 791 B1, EP 0 983 322 B1, EP 1 323 674 A1, EP 1 644 580 B1). Mixing of the components and precipitation of the extender are performed in a batch process in a vessel.
U.S. Pat. No. 3,528,838 discloses a method where precipitation is performed by reacting two dissolved reactants in the TiO2 suspension. In this case, TiO2 is dispersed in a sodium carbonate solution, and a calcium chloride solution is subsequently added, such that calcium carbonate is precipitated. This method is likewise performed as a batch process in a vessel.
U.S. Pat. No. 5,993,533 A, and also WO 2009/146834 A1, disclose a method for coating titanium dioxide by precipitating inorganic oxides, such as SiO2 and Al2O3, where a TiO2 suspension is mixed with the oxide precursor substance (first reaction component) in an inline mixer. The mixed suspension is subsequently pumped into a vessel, where the second reaction component (acid) is added, such that the reaction and precipitation can take place.